The instant invention relates to calibration of quadrature (I/Q) modulators, and in particular to self-calibration of a direct transmitter used in digital communication system, as well as in radar and instrumentation fields.
Digital communication techniques are by now the emerging standard in almost all areas of telecommunication. Digital communication is often based on a special class of communication signals, sent by a transmitter and received by a receiver, using digital modulation techniques. These techniques make use of a class of signal modulators, which perform vector or quadrature modulation. A vector-modulated or quadrature-modulated signal is an amplitude and phase modulated signal that is modulated according to in-phase and quadrature phase components, which define a phase vector. In vector or quadrature modulation, two modulation input signals independently modulate the in-phase (I) and quadrature (Q) components of a carrier signal. For proper operation, the I and Q channels of the modulator are ideally calibrated to be equal in gain, i.e. balanced, and 90xc2x0 apart, i.e. in quadrature.
It is known that gain and phase imbalance between the I and Q channels in a vector or quadrature modulator degrade the overall communication link performance. Although compensation within a quadrature receiver for eliminating the performance degradation due to the phase imbalance is effective, the degradation due to the gain imbalance is not to be eliminated at the receiver in a straightforward manner. In addition, any DC offsets possibly cause a local oscillator (LO) signal to leak through the vector modulator, the LO being part of the vector modulator. In many cases, vector modulators designed to be fed I- and Q-signals with a given DC offset, for example 0.5 V. After fabrication of the vector modulator, the optimal DC offset optionally takes on values that are different from the designed values. For example, the optimal DC offset for the I-channel is 0.48V, whereas the optimal offset for the Q-channel is 0.52 V. Therefore, the expression xe2x80x9ccorrecting for DC offsetxe2x80x9d has to be understood as xe2x80x9ccompensation for deviation from an optimal DC offsetxe2x80x9d.
The leakage as well as the gain and phase imbalance distort the transmitted signal, and make it difficult to meet a given spectrum mask requirement. Therefore, it is desirable to eliminate or minimize imbalances and deviations from optimum offsets at the transmitter. Preferably, a calibration process is used to achieve this task.
Given the importance of digital communication in today""s media world, a plethora of studies have been devoted to estimating and correcting for errors in vector modulators, and improving performance of direct transmitters. Therefore, the instant invention will be better understood and highly appreciated, when viewed in the context of relevant prior art.
Tuthill and Cantoni (1999 IEEE Int""l Conf. on Acoustics, Speech and Signal Processing, Vol. III, pp. 1621-1624) describe a method for compensation of the imperfections of reconstruction filters only. The proposed methodology is to adaptively control the digital channel filters to compensate for the reconstruction filter imperfections, such as departure from linear phase and from constant amplitude response, as well as gain and phase imbalance between I and Q filters. However, this methodology does not address the vector modulator imperfections.
Leyonhjelm and Faulkner (IEEE Trans. on Vehicular. Technology, Vol 44, No. 1, February 1995, pp. 95-102) address the use of a direct modulator for multiple channels. The phase and gain imbalances in transmit I and Q channels exhibit a frequency dependency, and thus require adjustment. The proposed technique is to add to each channel a lower side band (LSB) signal that will destructively add with the one generated. However, a channel has to be free, when the LSB signal to be inserted is determined, a frequency down conversion of the transmitted signal is also necessary.
Hilborn, Stapleton, and Cavers (IEEE Trans. on Vehicular Technology, Vol 43, No. 2, May 1994, pp. 223-233) propose a technique to compensate for both the quadrature modulator imperfections and the nonlinearities in power amplifiers. The technique to derive both quadrature modulator pre-distortion coefficients and amplifier linearizer coefficients is based on measuring the signal power in a channel adjacent to the desired one. It is shown that the power in the adjacent signal is approximately a quadratic function of the modulator and amplifier distortions. However, a pre-selection filter is needed at the output port to reject the transmitted signal and to look at an adjacent channel only. The technique works only when modulator and amplifier imperfections are small so that certain underlying approximations remain valid.
Cavers and Liao (IEEE Trans. on Vehicular Technology, Vol. 42, No. 4, November 1993, pp. 581-588.) propose a technique to compensate for direct modulator imperfections. A power detector is used to sample a modulator output signal, and by feeding the modulator with a known sequence of signals, coefficients for DC offset and gain and phase imbalance compensation are derived. This sequential technique adjusts first the DC offset and then the gain and phase imbalances. A series of tones with four different phases is used to derive the required gain and phase imbalance adjustment factors. A least-mean-square technique is used to average the coefficients obtained from several measurements, and to converge to a steady state. The adaptation and calibration is performed during non-transmission periods of time. In addition, the DC offset coefficients are calibrated by grounding the inputs of the vector modulator. In this case, measuring the output power of the modulator with a power detector is difficult, and the accuracy of the results is a function of sensitivity of a power detector and a dynamic range of a feedback system. Disadvantageously, it is necessary to make measurements with no signal fed to the vector modulator and thus requires a feedback loop (i.e. power detector and analog-to-digital converter) with high dynamic range operating during non-transmission periods of time.
Cavers and Liao (Proc. of IEEE Vehicular Technology Conference, May 19-22, 1991, pp. 578-583) propose a technique to be used at the receiver to compensate for both transmitter and receiver imperfections. A three-tap equalizer is used at the receiver in either a decision directed mode or reference aided mode. Although the authors claim that all the modulator imperfections are compensated, a modulator gain imbalance likely results in a signal-to-noise ratio degradation that is not likely recovered at the receiver. In addition, transmitter DC offset losses translate into local oscillator LO leakage which consumes transmit power inefficiently as opposed to power used for transmission of the desired signal. The quadrature signals are possibly recovered at the receiver even in the presence of modulator imperfections at a cost of a likely performance loss.
Cavers (IEEE Trans. on Vehicular Technology, Vol. 46, No. 3, August 1997, pp. 707-716) also derives pre-distortion coefficients from a solution of a set of linear equations. This technique is iterative, and usually uses two or more iterations to result in reasonable performance. The approximations made in solving the linear equations are only valid when errors are small. Further, the technique is computationally costly, and matrices are not likely to be well conditioned. Thus, a matrix inversion algorithm necessary is likely to be non trivial.
Ren and Wolff (1997 IEEE-MTT-S proceeding, Jun. 8-13, 1997, vol. 111, pp. 1691-1694) describe a technique to determine pre-distortion coefficients. The technique relies on demodulating the modulator""s output signal and building a database of correction factors for each potential I and Q signal to be transmitted. However, this method relies on a transmitter signal demodulated in a feedback loop.
Yang, Vos and Cho (Proc. of IEEE Vehicular Technology Conference, Atlanta, Apr. 28-May 1, 1996, Vol. II, pp. 1317-1320) address gain and phase imbalance effects in a direct modulator. They present a correction algorithm, which is not likely to be intended for real time implementations but as part of a calibration phase in a production line instead. Phase and gain imbalance pre-distortion factors are determined using a two-dimensional root finding algorithm, utilizing signal-to-image rejection ratio measured with a spectrum analyzer as a feedback signal. This method is not suitable for real time implementation nor does it compensate for DC offsets.
Lohtia, Goud, and Englefield (IEEE PACRIM 1993 Conference Proceedings, Victoria, May 19-21, 1993, Vol. II, pp. 447-450) defined a nonlinear model of the vector modulator and proposed a technique to assess five unknown model variables by solving a set of five independent nonlinear equations based on five output power measurements. The set of nonlinear equations is solved using a nonlinear root finding technique. However, nonlinear modeling and solving a set of nonlinear equations, possibly leads to convergence problems.
In U.S. Pat. No. 5,387,883, issued Feb. 7, 1995 to Ueda, a quadrature modulator having controlled phase shifter is described. However, the modulator compensates only for phase imbalance. The modulator is fed with calibration signals during non-transmission periods of time.
In U.S. Pat. No. 5,293,406, issued Mar. 8, 1994 to Suzuki, a quadrature amplitude modulator with distortion compensation is disclosed. Again, the modulator works during non-transmission periods of time, and relies on the use of various calibration signals. Furthermore, sequential compensation is not likely to warrant full compensation due to a correlation between gain and phase imbalances.
In U.S. Pat. No. 5,663,691, issued Sep. 2, 1997, to Kowalik et al., an estimator for estimating an operating defect in a quadrature modulator, and a modulation stage using the estimator, are described. In this approach, the instantaneous output signal power is multiplied with the base band I and Q signals, the product is integrated to get an indication of the phase imbalances and DC offsets. However, there is no amplitude imbalance compensation and/or estimation. Most likely this technique utilizes a manual calibration process.
It would be highly advantageous to provide a system and method that overcome the limitations and drawbacks of the prior art.
It is an object of the instant invention to provide a calibration method of determining pre-distortion coefficients to be applied to in-phase (I) and quadrature (Q) signals feeding a direct transmitter in order to maximize local oscillator (LO) suppression, as well as to maximize unwanted sideband suppression, the pre-distortion coefficients being used to adjust the relative phase and gain between the I and Q signals as well as DC offsets of the I and Q signals, the proposed technique not involving any approximation on the values of the pre-distortion coefficients.
It is also an object of the instant invention to provide a calibration method that is fully adaptive, thus allowing the use of a low dynamic range in the feedback circuit.
It is further an object of the instant invention to provide a calibration method that is able to track and correct changes over times as they occur.
It is another object of the instant invention to provide a calibration method for adaptive determination of gain and phase imbalances and DC offsets, using a least-mean-square (LMS) or recursive-least-square (RLS) based process, and using a truly mapped linear model without any approximation.
It is further another object of the instant invention to provide a calibration method for adaptive estimation based on an algorithm, which does not rely on contiguous samples of the output signal power, thus allowing for a low sampling rate.
It is yet another object of the instant invention to provide an apparatus for providing a carrier modulated signal, in which the I and Q channel signals have been pre-distorted for local oscillator as well as unwanted sideband signal suppression the pre-distortion coefficients obtained applying an algorithm based on a truly mapped linear model without any approximations.
In accordance with an aspect of the instant invention, there is provided a method for self-calibrating a vector modulator. The method includes a step of determining pre-distortion coefficients in dependence upon an in-phase signal, a quadrature signal, and an output envelope of an RF signal. The method further comprises the steps of transforming a value representative of an output envelope represented in a nonlinear domain into a value representative of the output envelope represented in a linear domain, determining a parameter vector x as a solution of a linear equation within the linear domain, and determining the pre-distortion coefficients from the parameter vector x.
In accordance with another aspect of the instant invention, there is provided a compensating quadrature modulator for self-calibrating a vector modulation process, the compensating quadrature modulator comprising: an I-signal transmit chain for pre-distorting and transmitting an in-phase signal, a Q-signal transmit chain for pre-distorting and transmitting a quadrature signal, a vector modulator for modulating the in-phase signal and the quadrature signal and for providing an analog RF output signal, an envelope detector coupled to the vector modulator output for detecting the analog RF output signal and for providing an analog envelope of the RF output signal, an analog-to-digital converter coupled to the envelope detector for receiving the analog envelope RF output signal and for converting the received signal into a digital envelope of an RF output signal, and a digital signal processor for determining at least a digital signal pre-distortion coefficient by which signals in one of the I-signal transmit chain and the Q-signal transmit chain are pre-distorted, the digital signal processor being coupled to the analog-to-digital converter, and being coupled to the I-signal transmit chain and the Q-signal transmit chain. The digital signal processor comprises circuitry for performing the steps of transforming a value representative of an output envelope represented in a nonlinear domain into a value representative of the output envelope represented in a linear domain, determining a parameter vector x as a solution of a linear equation within the linear domain, and determining the pre-distortion coefficients from the parameter vector x.